In the high-voltage and high-power field, multi-level electric and electronic convertors are more and more widely used, owing to their unique advantages. A modular multi-level convertor is a new voltage source type convertor, which is designed with a strict modular structure, and thereby the production cost can be reduced in mass production; by submodules connected in series, the voltage class and power class of a convertor are easy to expand; owing to the multi-level output form of such a convertor, the harmonic content and total harmonic distortion of the output voltage are decreased, and thereby high-capacity AC filters can be reduced or even omitted; since the submodules in the bridge arms do not have to be switched on simultaneously, the variation rates of voltage and current in the bridge arms can be decreased, so that the stress borne on the switching elements is greatly decreased; moreover, the protective circuit of a modular multi-level convertor is simple and easy to implement.
The modulation policies for multi-level electric and electronic convertors mainly include Carrier Based Pulse Width Modulation (CBPWM) policies and Space Vector Based Pulse Width Modulation (SVPWM) policies, etc. SVPWM policies are rarely used in convertors that involve three or more levels, because they involve very complex calculation and it is difficult to select redundant vectors as the number of output levels increases. CBPWM policies are used widely. At present, most CBPWM policies for modular multi-level convertors utilize a carrier phase shift technique. When a carrier phase shift based pulse width modulation policy is used, the upper bridge arm and lower bridge arm of each phase require one modulation wave and N carrier waves that are phase-shifted in sequence, and the harmonic content of the output voltage can be greatly reduced without increasing the switching frequency. The convertor is easy to modular implement, and the workload on the switching elements is balanced and uniform. However, the workload in the control system will be increased when a carrier phase shift based pulse width modulation policy is used, owing to the fact that each carrier wave obtained by phase shift has to be generated separately and the carrier waves have to be synchronized. Especially, for a convertor that involves a large number of levels and a large quantity of submodules, a large quantity of carrier waves have to be generated when such a modulation policy is used. Thus, the requirement for software/hardware resources of the system is very stringent. Compared with carrier phase shift based pulse width modulation policies, to attain the same power output quality, the modulation policy for a modular multi-level convertor provided in the present invention does not require phase shift control of carrier waves, and each phase only requires one modulation wave and N carrier waves, which is to say, the required quantities of modulation waves and carrier waves are halved; in addition, with the modulation policy disclosed in the present invention, it is unnecessary to design a capacitive voltage balance closed-loop control algorithm for submodules; thus, the software/hardware resources of the system can be saved significantly, and engineering realization is easy. Hence, the modulation policy disclosed in the present invention is more suitable for use in modular multi-level convertors that involve a large number of levels.